Orban OPTIMOD-FM 8700, Operating Manual

Page: 33 / 390

OPTIMOD-FM DIGITAL INTRODUCTION
1-3
This produces a clean, open sound whose subjective brightness more closely
matches the original program.

The 8700i achieves extremely tight peak control at all its outputs—analog, AES3
(for both the analog FM and HD channels), and composite baseband.

The stereo encoder has two outputs with independent level controls, each capa-
ble of driving 7

in parallel with 47,000pF, (100ft / 30m of coaxial cable).

By integrating the stereo encoder with the audio processing, the 8700i elimi-
nates the overshoot problems that waste valuable modulation in traditional ex-
ternal encoders.

The 8700i prevents aliasing distortion in subsequent stereo encoders or transmis-
sion links by providing bandwidth limiting and overshoot compensated 15 kHz
low-pass filters ahead of the 8700i’s audio outputs and stereo encoder.

The 8700i has an internal, DSP-based stereo encoder (with a patented “half-
cosine interpolation” composite limiter operating at 512 kHz sample rate) to
generate the pilot tone stereo baseband signal and control its peak level. The
composite limiter is a unique, “you can only do this in DSP” process that beats
composite clippers by preserving stereo imaging while fully protecting the stereo
pilot tone, RDS/RBDS, and subcarriers.

The stereo encoder’s stereo subchannel modulator can operate in normal double
sideband mode and in an experimental compatible single sideband mode that is
offered to enable users to compare and assess the two modes. See
SSB Stereo
Encoder Operation on page 3-12.

The Digital Radio and analog radio processing chains offer ITU-R BS.1770-3+
Loudness Meters and Safety Limiters for use in countries that enforce a BS.1770
loudness limit on digital and/or analog radio broadcasts.

The 8700i implements “true peak” control in the HD processing chain by over-
sampling the HD peak limiter’s sidechain at 256 kHz. This allows the 8700i to
prevent clipping in a playback device’s analog signal path by predicting and con-
trolling the analog peak level following the playback device’s reconstruction fil-
ter to an accuracy of better than 0.5 dB. For typical program material, accuracy is
0.2 dB
Without true peak control, analog clipping can occur even if all peak values of
the digital samples are below 0 dBFS. This phenomenon has also been termed
“0 dBFS+.”
Thanks to true peak control, sample rate conversion, unless it removes high fre-
quency program energy or introduces group delay distortion, cannot cause sam-
ple peaks to increase more than 0.5 dB. For example, sample rate conversion
from 48 kHz to 44.1 kHz is highly unlikely to cause sample peak clipping in the
44.1 kHz audio data.